To delete all categories in Magento about sql you can use the following query.

If you has more than 3 stores, you must extend the entry „catalog_product_flat_1;“ with your store-id, e.g.:

DELETE FROM catalog_product_flat_3;
DELETE FROM catalog_product_flat_4;

and so on …

SET FOREIGN_KEY_CHECKS = 0;
TRUNCATE TABLE `catalog_category_entity`;
TRUNCATE TABLE `catalog_product_bundle_option`;
TRUNCATE TABLE `catalog_product_bundle_option_value`;
TRUNCATE TABLE `catalog_product_bundle_selection`;
TRUNCATE TABLE `catalog_product_entity_datetime`;
TRUNCATE TABLE `catalog_product_entity_decimal`;
TRUNCATE TABLE `catalog_product_entity_gallery`;
TRUNCATE TABLE `catalog_product_entity_int`;
TRUNCATE TABLE `catalog_product_entity_media_gallery`;
TRUNCATE TABLE `catalog_product_entity_media_gallery_value`;
TRUNCATE TABLE `catalog_product_entity_text`;
TRUNCATE TABLE `catalog_product_entity_tier_price`;
TRUNCATE TABLE `catalog_product_entity_varchar`;
TRUNCATE TABLE `catalog_product_link`;
TRUNCATE TABLE `catalog_product_link_attribute`;
TRUNCATE TABLE `catalog_product_link_attribute_decimal`;
TRUNCATE TABLE `catalog_product_link_attribute_int`;
TRUNCATE TABLE `catalog_product_link_attribute_varchar`;
TRUNCATE TABLE `catalog_product_link_type`;
TRUNCATE TABLE `catalog_product_option`;
TRUNCATE TABLE `catalog_product_option_price`;
TRUNCATE TABLE `catalog_product_option_title`;
TRUNCATE TABLE `catalog_product_option_type_price`;
TRUNCATE TABLE `catalog_product_option_type_title`;
TRUNCATE TABLE `catalog_product_option_type_value`;
TRUNCATE TABLE `catalog_product_super_attribute`;
TRUNCATE TABLE `catalog_product_super_attribute_label`;
TRUNCATE TABLE `catalog_product_super_attribute_pricing`;
TRUNCATE TABLE `catalog_product_super_link`;
TRUNCATE TABLE `catalog_product_enabled_index`;
TRUNCATE TABLE `catalog_product_website`;
TRUNCATE TABLE `catalog_product_entity`;
TRUNCATE TABLE `cataloginventory_stock`;
TRUNCATE TABLE `cataloginventory_stock_item`;
TRUNCATE TABLE `cataloginventory_stock_status`;
TRUNCATE TABLE `catalog_product_link`;
TRUNCATE TABLE `catalog_product_link_type`;
TRUNCATE TABLE `catalog_product_option`;
TRUNCATE TABLE `catalog_product_option_type_value`;
TRUNCATE TABLE `catalog_product_super_attribute`;
TRUNCATE TABLE `catalog_product_entity`;
TRUNCATE TABLE `cataloginventory_stock`;
TRUNCATE TABLE `catalog_category_product`;
DELETE FROM catalog_product_flat_1;
DELETE FROM catalog_product_flat_2;
DELETE FROM catalog_product_flat_3;
SET FOREIGN_KEY_CHECKS = 1;
insert  into `catalog_product_link_type`(`link_type_id`,`code`) values (1,'relation'),(2,'bundle'),(3,'super'),(4,'up_sell'),(5,'cross_sell');
insert  into `catalog_product_link_attribute`(`product_link_attribute_id`,`link_type_id`,`product_link_attribute_code`,`data_type`) values (1,2,'qty','decimal'),(2,1,'position','int'),(3,4,'position','int'),(4,5,'position','int'),(6,1,'qty','decimal'),(7,3,'position','int'),(8,3,'qty','decimal');
insert  into `cataloginventory_stock`(`stock_id`,`stock_name`) values (1,'Default');

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Kategorie neu laden anhand vorhandener Kategorie-ID

$category = Mage::getModel('catalog/category')->load(12);

Unterkategorien der Kategorie erhalten

$category->getChildren();

Beliebige Werte der aktuellen kategorie erhaltet ihr mit z.b.

$category->getId(); // the cat id
$category->getName(); // cat name
$category->getLevel(); // cat level
$category->getParentCategories(); // get parent categories

Den Kategorien Pfad im Store zur aktuellen Kategorie anzeigen lassen geht mit

$category->getPathInStore();

Alle Hauptkategorien (unterhalb der 0-Level-Default-Kategorie) anzeigen lassen

    public function getParentTopCategory($category = null)
    {
        if(is_null($category)) {
            return $category;
        }
        if($category->getLevel() == 2){
            return $category;
        } else {
            $parentCategory = Mage::getModel('catalog/category')->load($category->getParentId());
            return $this->getParentTopCategory($parentCategory);
        }
    }